The anatomy and physiology of spinomesencephalic tract (SMT) cells will be studied using retrograde labeling and extracellular recording techniques, respectively. Spinal input to the mesencephalon has been documented in a variety of species including man. Projections to this region ascend in the ventral spinal cord along with the spinoreticular and spinothalamic tracts. While it is well established that the spinothalamic and spinoreticular projections are involved in the transmission of nociceptive information, by comparison little is known about the spinomesencephalic tract. Based on available clinical and experimental data this pathway is generally thought to be an important component in the neuronal apparatus responsible for the perception of pain. The present research is designed to examine the anatomical and functional organization of this pathway. To this end, retrograde tracing techniques will be used to determine the distribution of spinal neurons projecting to different midbrain regions in the rat, cat and primate. Particular attention will be given to the distribution of cells projecting to the deep layers of the superior colliculus and to areas known to be involved in the descending control of dorsal horn neurons or in the production of stimulation produced analgesia. Since the cells of origin of the SMT are thought to have an overlapping distribution with spinothalamic and spinoreticular tract cells, the possibility exists that the ascending axons of SMT cells may distribute collaterals to more than one postsynaptic target. To evaluate this question, experiments using two different retrograde labels delivered to non-overlapping injection sites will be carried out initially in the rat and if warranted in the cat and primate. The functional properties of the SMT will be studied using electrophysiological techniques to examine the afferent convergance and receptive field organization of identified SMT cells in the primate. The response of these cells to noxious/innocuous mechanical and thermal stimuli will be evaluated. These studies should provide further insight into the organization of ascending spinal pathways responsible for the transmission of sensory information. Moreover, it is felt that an important step towards understanding the interaction between ascending sensory pathways and descending control systems is to gain a better understanding of the afferent input to these systems.